Learn French: Elle a fait une tarte aux cerises.

When naming a dish characterized by a specific flavor or added ingredient, French uses the preposition à (which combines with articles to make au, à la, or aux). You use de when something is made entirely of that material, like jus de cerise (cherry juice). Since a tart is a pastry that features cherries rather than being made of them, it takes à + les = aux.

Why is cerises plural here (aux cerises), but singular in something like une tarte au citron (a lemon tart)?

French usually makes the ingredient plural if you can easily count the individual pieces used in the recipe, like cherries (cerises), strawberries (fraises), or apples (pommes). If the ingredient is treated as an uncountable flavor or a mass, like lemon (citron) or chocolate (chocolat), it takes the singular.

Could I say Elle a cuisiné une tarte instead of Elle a fait?

It is much more natural to use fait or préparé. In French, cuisiner refers to the general act or hobby of cooking. When talking about putting together a specific dish or baked good, native speakers almost always use faire (to make) or préparer (to prepare).